Rename table without downtime

With our database helper methods built into GitLab, it's possible to rename a database table without downtime.

The technique builds on top of database views, using the following steps:

Rename the database table.
Create a database view using the old table name by pointing to the new table name.
Add workaround for ActiveRecord's schema cache.

For example, consider that we are renaming the issues table name to tickets. Run:

BEGIN;
  ALTER TABLE issues RENAME TO tickets;
  CREATE VIEW issues AS SELECT * FROM tickets;
COMMIT;

As database views do not expose the underlying table schema (default values, not null constraints, and indexes), we need further steps to update the application to use the new table name. ActiveRecord heavily relies on this data, for example, to initialize new models.

To work around this limitation, we need to tell ActiveRecord to acquire this information from a different table using the new table name.

Migration strategy breakdown

Release N.M: Mark the ActiveRecord model's table

Consider the current release as "Release N.M".

In this release, register the database table so that it instructs ActiveRecord to fetch the database table information (for SchemaCache) using the new table name (if it's present). Otherwise, fall back to the old table name. This is necessary to avoid errors during a zero-downtime deployment.

Edit the TABLES_TO_BE_RENAMED constant in: lib/gitlab/database.rb

TABLES_TO_BE_RENAMED = {
  'issues' => 'tickets'
}.freeze

Note that, in this release (N.M), the tickets database table does not exist yet. This step is preparing for the actual table rename in release N.M+1.

Release N.M+1: Rename the database table

Consider the next release as "Release N.M".

Execute a standard migration (not a post-migration):

  def up
    rename_table_safely(:issues, :tickets)
  end

  def down
    undo_rename_table_safely(:issues, :tickets)
  end

Important notes:

Let other developers know that the table is going to be renamed.
- Ping the @gl-database group in your merge request.
- Add a note in the Engineering Week-in-Review document: table_name is going to be renamed in N.M. Modifications to this table are not allowed in release N.M and N.M+1.
The helper method uses the standard rename_table helper from Rails for renaming the table.
The helper renames the sequence and the indexes. Sometimes it diverges from the standard Rails convention when naming indexes, so there is a possibility that not all indexes are properly renamed. After running the migration locally, check if there are inconsistently named indexes (db/structure.sql). Those can be renamed manually in a separate migration, which can be also part of the release M.N+1.
Foreign key columns might still contain the old table name. For smaller tables, follow our standard column rename process
Avoid renaming database tables which are using with triggers.
Table modifications (add or remove columns) are not allowed during the rename process. Make sure that all changes to the table happen before the rename migration is started (or in the next release).
As the index names might change, verify that the model does not use bulk insert (for example, insert_all and upsert_all) with the unique_by: index_name option. Renaming an index while using these methods may break functionality.
Modify the model code to point to the new database table. Do this by renaming the model directly or setting the self.table_name variable.

At this point, we don't have applications using the old database table name in their queries.

Remove the database view through a post-migration:

  def up
    finalize_table_rename(:issues, :tickets)
  end

  def down
    undo_finalize_table_rename(:issues, :tickets)
  end

Additionally the table definition from TABLES_TO_BE_RENAMED must be removed.

To do so, edit the TABLES_TO_BE_RENAMED constant in lib/gitlab/database.rb:

From:
```
TABLES_TO_BE_RENAMED = {
  'issues' => 'tickets'
}.freeze
```
To:
```
TABLES_TO_BE_RENAMED = {}.freeze
```

Zero-downtime deployments

When the application is upgraded without downtime, there can be application instances running the old code. The old code still references the old database table. The queries still function without any problems, because the backward-compatible database view is in place.

In case the old version of the application needs to be restarted or reconnected to the database, ActiveRecord fetches the column information again. At this time, our previously marked table (TABLES_TO_BE_RENAMED) instructs ActiveRecord to use the new database table name when fetching the database table information.

The new version of the application uses the new database table.